This invention relates to a corrosion-resistant porous metallic member whose pores communicate with each other and which can be used as a material for various kinds of filters, especially corrosion-resistant, heat-resistant filters and catalyst carriers, and a method of manufacturing the same.
Unexamined Japanese Patent Publications 1-255686 and 63-81767 disclose pure-nickel porous members which are used as materials for battery electrodes. The methods for manufacturing such porous members disclosed in these publications comprise the steps of depositing a metal by electroplating on a conductive unwoven fabric or an unwoven fabric subjected to conductivity-imparting treatment, and heating the plated fabric to remove the fabric core body and at the same time increase the density of the metal structure. Examined Japanese Patent Publications 42-13077 and 54-42703 disclose stainless porous filter members manufactured by forming an unwoven fabric of metallic fibers obtained by drawing and cutting, and then sintering it.
In the method disclosed in the first publication, a metal layer is formed by electroplating on a conductive, three-dimensional, reticular, porous resin substrate by bringing it into tight contact with a cathode in a plating bath, the cathode being in the form of exposed spots studded on a conductor which is insulated except its exposed cathode spots.
The metallic porous member formed by this method has a balanced weight distrubution in its thickness direction. Before this method was developed, it was impossible to provide a metallic porous members having such a uniform weight distribution in a thickness direction.
The battery electrode disclosed in the second publication is manufactured by the steps of: impart ing conductivity to a strip of non-conductive resin or unwoven fabric having a three-dimensional reticular structure; moving the strip as a cathode in a plating bath while pressing its one side against a feed electrode to form a secondary conductive layer in the form of a metal plated layer on the surface of the strip; forming metal plated layers of a predetermined thickness on both sides of the strip as a cathode, cutting the strip to a predetermined shape, and winding the strip with its side pressed against the feed electrode in the plating bath facing inside.
Before this publication, it was difficult to provide a uniform electrocoating layer in the pores of a non-conductive porous member due to a difference in current density between its surface and inner portion. This publication tried to solve this problem.
The third publication discloses a method of manufacturing a filter element, which comprises the stepsof drawing a metal wire to an extremely small diameter, annealing it in a furnace kept in a non-oxidizing atmosphere, cutting it to a suitable lengths, forming the thus cut wires into an unwoven fabric, and sintering the fabric under pressure in a reducing atmosphere.
This publication aims to provide a filter element which has high shock resistance and strength and which can be manufactured with a smaller number of steps.
The fourth publication discloses a method of manufacturing a reinforced metal filter. In this method, a reinforced metal filter is formed by placing a mass of square stainless steel filaments in an oxygen-free atmosphere or in a vacuum, compressing the entire mass flatly at a constant pressure while heating it to collapse the filaments along the ridgelines of the joint portions between the filaments and thus to partially increase the joint area corresponding to the pressure applied, and hardening the entire mass while controlling the area of the pores formed between the filaments due to intermetallic diffusion at joint area.
This publication aims to reduce the number of manufacturing steps and provide a product high in heat efficiency while suitably controlling the porosity of the filter member.
In the first method, only a limited kinds of metals can be deposited by plating. It is impossible to form a sufficiently corrosion-resistant and heat-resistant alloy which can withstand a temperature of more than 500.degree. C., such as Ni--Cr or Ni--Cr--Al alloy, which the applicant of this invention proposed in Unexamined Japanese Patent Publication 5-206255), or Fe--Cr or Fe--Cr--Al alloy, which is now gathering attention as materials for catalyst carriers for treating gasoline engine emissions. In the second method, it is impossible to form metal fiber. Thus, the article obtained in this method loses its heat resistance and corrosion resistance at 600.degree. C. or over.
In order to solve the problems of these two methods, it has been proposed to use these two methods in conjunction with what is known as a powder diffusion method for preparing an alloy composition which is used to provide a corrosion-resistant coating on a car body or the like. Namely, in this method, a metallic porous member prepared by either of the above two methods is buried in a powder containing Al, Cr and NH.sub.4 Cl, and heated at 800.degree.-1100.degree. C. to adjust the alloy composition by depositing and diffusing Cr and Al to obtain a sufficiently heat-resistant and corrosion-resistant alloy.
If the mutually communicating pores in the alloy thus formed have a diameter smaller than 100 .mu.m, the distribution of composition of the porous member tends to be large in a thickness direction. If its thickness is 1 mm or more, the content at its center with respect to the thickness direction may be one-tenth or less of the content at its outermost area. If the Cr and/or Al content is increased to increase the heat resistance and corrosion resistance so that the alloy can withstand a temperature of 700.degree. C. or higher even at its central portion, the toughness of the alloy tends to be low. This impairs the formability and resistance to vibration, which will, after all, makes it impossible to obtain a heat-resistant and corrosion-resistant material which can withstand a temperature higher than 700.degree. C.
Another problem with Ni--Cr--Al alloy and Fe--Cr--Al alloy is that if the amount of Al is increased to increase the heat resistance of the alloy, its toughness tends to decrease correspondingly, thus lowering formability. This makes it necessary to adjust the alloy composition after forming a metallic porous member made of Ni, Fe, Ni--Cr or Fe--Cr into a predetermined shape. According to the final shape of the porous member, it may be necessary to use a technique for diffusing components uniformly in the thickness direction. But if the metallic porous member is alloyed with Cr and Al simultaneously by the powder diffusion method, in which Cr and Al powders are mixed, the Cr content tends to be insufficient since the vapor pressure of Cr is lower than that of Al. Also, the Cr content tends to be uneven, especially in the thickness direction. The metallic member thus formed tends to be too low in corrosion resistance at its central portion.
An object of the present invention is to provide a heat-resistant, corrosion-resistant metallic porous member which is free of these problems and a method of manufacturing such a porous member.